1. Field of the Invention
The invention concerns active roll and yaw control of a satellite with onboard kinetic moment in equatorial or near-equatorial (less than 10.degree. inclination) orbit stabilized about three axes.
2. Description of the Prior Art
There are various known systems of attitude control for three-axis stabilized satellites with onboard kinetic moment, in other words embodying one or more momentum wheels whose resultant kinetic moment always has a non-null value, so generating a gyroscopic stabilization effect.
For example, French Patent No. 2,522,614 (MOUILHAYRAT et al of CENTRE NATIONALE D'ETUDES SPATIALES) concerns a satellite having a platform stabilized to face the Sun and a payload mounted to rotate about a North-South axis relative to the platform so as to remain stabilized with reference to the Earth. To generate attitude correction torques the platform contains actuators in the form of a momentum wheel with its axis perpendicular to the North-South axis and a reaction wheel; their rotation speeds are controlled according to information on the attitude of the platform provided by solar and stellar sensors. Rotation of the payload relative to the platform is controlled according to the output of a terrestrial sensor which does not cooperate with the solar and stellar sensors.
French Patent No. 2,637,565 (MAUTE of AEROSPATIALE) describes an attitude control system for geostationary satellites using signals from terrestrial, solar and stellar sensors to control actuators which can be reaction wheels, momentum wheels or thrusters. A number of processing lines are provided in parallel, representing various modes of operation.
French Patent No. 2,319,150 (MUHLFELDER and SCHMIDT of RCA CORPORATION) concerns a satellite in synchronous orbit with fixed kinetic moment oriented on the pitch axis and where the requirement is to dispense with any measurement of attitude in yaw. Only the roll attitude is measured (using a horizon sensor) and attitude correction is applied if the attitude in roll is incorrect by an amount exceeding a threshold (binary triggering) using a magnetic coil in the roll-yaw plane interacting with the terrestrial magnetic field.
French Patent No. 2,498,155 (MUHLFELDER, PHILIPS and BALSNIK of RCA CORPORATION) describes an improvement to the just previously mentioned patent introducing a time-delay into the generation of the magnetically sourced correcting torque determined according to the period of nutation of the satellite.
The U.S. Pat. NO. 4,294,420 (BROQUET of MATRA) describes a satellite having two momentum wheels in a V configuration in a plane containing the pitch axis and rotation of which is controlled according to attitude measurements in roll and in pitch supplied by terrestrial sensors. The kinetic moment can be oriented with one degree of freedom. The attitude is stabilized in yaw by passive means in the short term, using gyroscopic techniques, and possibly complemented by the use of thrusters or magnetic coils for long-term corrections.
U.S. Pat. No. 4,521,855 (LEHNER and LEBSOCK of FORD AEROSPACE) concerns roll-yaw control of a satellite which explicitly dispenses with a stellar sensor and uses a kinetic moment which can be oriented in the pitch/yaw plane. Only the attitude in roll is measured; a high-speed roll loop determines variations in the speed of the wheels and a low-speed yaw loop (of the LUENBERGER type) controls a single magnetic coil on the yaw axis.
The article published in Proceedings of AOCS conference, Noordwijk, Netherlands, 3-6 October 1977, ESA SP-128 November 1977, pp. 103-110 (LACOMBE J. L.) under the title "Magnetotorquing for the attitude control of geostationary satellites" describes a method of designing a magnetic roll-yaw control system for three-axis stabilized geosynchronous satellites using two wheels in a V configuration and proposes to generate the onboard magnetic dipole according to a law based on the well-known WHECON control method used with thrusters. To this end the axis of the coil is offset to the yaw axis by an angle .alpha. approaching 45.degree. and the modulus of the magnetic dipole generated is determined by the roll angle as measured by a terrestrial sensor.
The article published in Journal of Guidance and Control, Vol. 2; no 4 - July-August 79, pp. 334-338 (GOEL P. L. and RAJARAM S.) under the title "Magnetic attitude control of a momentum-biased satellite in near equatorial orbit" describes a closed loop roll/yaw magnetic control system for a three-axis stabilized satellite with fixed kinetic moment. The roll error, measured by a terrestrial sensor, is filtered to provide two components: a component at the orbital angular frequency and a component at the nutation angular frequency. A magnetic coil on the roll axis is then energized so that its magnetic dipole is a linear function of these two components to provide simultaneously short-term and long-term attitude control.
The article published in Acta Astronautica, Vol 9, no 12, 1982, pp. 697-702 under the title "Advanced reaction wheel regulator for attitude control of spacecraft" (TSUCHIYA K., INOUE M., WAKASUQI N. and YAMAGUCHI T.) describes an original law for short-term roll/yaw control in the case of a satellite having a pitch momentum wheel and a yaw reaction wheel. In this system the reaction wheel control law implements the modern state variable control technique to the roll angle as measured by the Earth sensor and to the roll and yaw angular speeds as estimated by means of a minimal second order Luenberger sensor. The only attitude measurements are provided by a terrestrial sensor.
Finally, HUBERT and BRUNO in "Nutation damping using a pivotal momentum wheel", an article published in: Journal of Guidance, Control and Dynamics, Vol. 12, no 5, Sept.-Oct. 89, pp.756-757, describe a system for damping nutation of the pitch axis (or for short-term roll/yaw control) for a satellite having a kinetic momentum that can be oriented by means of a momentum wheel on a pivot. Pivoting about an axis parallel to the roll axis is actuated by a stepper motor controlled by non-linear logic sensing the change of sign of the roll angular speed. The latter speed is measured by means of a rate gyro or calculated by differentiating the roll angle as measured by an Earth sensor. The signal obtained is then filtered by a bandpass filter centered on the satellite nutation angular frequency.
An object of the invention is to offer improved performance as compared with the above solutions, that is durable in time and has no excessive penalty in terms of cost or mass.
The invention is directed to attitude control of three-axis stabilized satellites with onboard kinetic moment in equatorial or near-equatorial orbit that is highly accurate in yaw and in roll, for example to within 0.02.degree., has a moderate mass, is simple and of reasonable cost, and requires no significant consumption of energy (especially in the form of propellants), whereby it meets in particular requirements for pointing of future optical telecommunications satellites or second generation direct TV broadcast satellites.